ABSTRACT: We studied the patterns of waterborne metabolites released by diatom cultures during different growth phases. A carefully optimized enrichment procedure for metabolites from seawater medium facilitated the generation of large-scale sample sets that could be used for metabolic profiling. Samples from cartridge enrichments were directly submitted to ultraperformance liquid chromatography (UPLC) coupled with time-of-flight mass spectrometry (TOF-MS). Metabolic profiling revealed distinct patterns of metabolite release depending on the growth phases of the major bloom-forming diatoms Skeletonema marinoi and Thalassiosira pseudonana. Principal component analysis (PCA) enabled the recognition of specific metabolites that were only temporarily released by growing algal cultures. The exometabolome of the cells was complex and variable. The kinetics of metabolite release of some compounds resembles those of signal molecules involved in quorum sensing by bacteria. The introduced exometabolome investigation will help unravel the implications of release patterns of potential infochemicals for plankton ecology.